The present invention relates generally to the field of test apparatus, and more particularly to the field of test apparatus for sealing devices.
High pressure oxygen environments are inherently dangerous. Events that normally bring only mild consequences can assume catastrophic proportions, because the high pressure and attendant high temperatures can easily induce combustion, and the enriched oxygen environment rapidly turns combustion into catastrophe. That is exactly the case with seal members used in liquid oxygen (LOX) or pressurized gaseous oxygen (GOX) systems. At normal pressures, or in atmospheric air, failure of a seal member may lead to system shutdown, but that is generally all. The same event in a LOX or GOX system, however, can lead to destruction of the entire system, as failure causes the seal to ignite, which in turn sparks a combustion chain that includes the entire pump or other apparatus.
A major difficulty in designing equipment for LOX or GOX service is that current test procedures simply do not indicate whether a given seal element will adequately perform in a given system. The standard test for determining the suitability of a seal member in LOX or GOX service is ASTM Test Method G 86, published by the American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103, entitled "Standard Test Method for Determining Ignition Sensitivity of Materials to Mechanical Impact in Pressurized Oxygen Environments." This test subjects a test item to a mechanical impact with a striker pin dropped from a predetermined height, within a chamber in which a LOX or GOX atmosphere has been introduced.
That test method, however, has little relation to the actual loads experienced by a seal member in service. It is immediately apparent, moreover, that this test has nothing whatsoever to do with the ability of a seal member to perform its primary function--sealing. The test only addresses the response of a material to a single impact. In service, however, a valve seal member experiences not one but many impacts, often at high cyclic rates. Other seals, such as O-rings, never experience impact loading but rather are subject to friction loads that can abrade material and cause its temperature to rise.
Also completely ignored by this test method are adiabatic compression effects. Within a high-pressure, relatively low-volume environment such as a LOX or GOX system, the pressure waves that result from ordinary actions such as shutting a valve can have disastrous effects, because the increased pressure causes an adiabatic temperature rise in the fluid, and because the compression wave propagates rapidly through the system, the temperature rise is effectively instantaneous. In a 10,000 psi system, for example, such a pressure wave can produce a temperature of 1000.degree. F. in the medium surrounding a seal member. This sort of thermal load is significantly different from anything revealed by the ASTM test, and can cause seals that seemed perfectly suitable to fail. Several catastrophic system failures have been traced to this phenomenon.
The art has so far been unable to devise an effective, acceptable test procedure for seal members. For example, U.S. Pat. No. 5,000,033, issued to Turner on Mar. 19, 1991 and entitled "O-ring Gasket Test Fixture" does address the problem of testing O-rings, but it does not look to the special problems of a LOX/GOX environment. Thus, there is no attempt to replicate actual service conditions; a standard test fixture is deemed sufficient for all tested items. Also, no provision is made for monitoring temperature changes of the material during use, a primary failure mode for O-rings in LOX/GOX service. And, of course, there is no provision whatsoever for testing any seal members other than O-rings. Other disclosures of seal testing apparatus and methods simply fail to grapple with the particular problems of LOX/GOX systems. These references include U.S. Pat. Nos. 4,903,529, (Hodge, Feb. 27, 1990, entitled "Valve System Analyzer"); 3,400,572 (Mizenko, Sep. 10, 1968, entitled "Seal Evaluation Test Fixture"); and 3,213,674 (Salcido, Oct. 26, 1965, entitled "Tool").
The primary result of this failure has been increased development time and cost, because cost effective seal materials consistently fail the ASTM G 86 test procedure at elevated pressures. It has been observed, for example, that tests performed on standard materials that are known to perform in actual applications at pressures up to 10,000 psi do not pass the existing test, owing to the differences in load characteristics. Faced with such a situation, engineers must either select a different seal material or seek an exception to the test procedure. If the latter course is chosen, the available alternative materials are more expensive by at least an order of magnitude, so that a seal that should cost in the range of about $5 costs about $50. Needless to say, the total cost of such an increase, when spread over an entire system, is considerable. Even so, however, situations exist in which no existing material can pass the ASTM test, despite the fact that engineers know that certain readily available seals will perform perfectly adequately. Thus, time, effort and money must be invested in justifying an exception to the test procedure, which drives up development cost and stretches out development time.
A need exists for a simple, straightforward method for testing seal members in a fashion that can yield dependable results. That goal is achieved in the present invention.